The invention relates to a low-pressure mercury vapour discharge lamp having a very satisfactory color rendition, having color temperature of the emitted white light of at least 3200 K. and having a color point (x.sub.L,y.sub.L) at or near the Planckian locus, which is provided with a gas-tight radiation-transparent envelope containing mercury and rare gas and with a luminescent layer containing a luminescent halophosphate and a luminescent material activated by bivalent europium.
The expression "a very satisfactory color rendition" is to be understood in this description and the appended claims to mean that the average color rendering index R(a,8) (average value of the rendering indices of eight test colors, as defined by the Commission Internationale d'Eclairage: Publication CIE No. 13.2 (TC-3.2), 1974), has a value of at least 85.
The color of visible radiation is characterized by the color co-ordinates (x,y) which determine the color point in the color triangle (see Publication CIE No. 15 (E-1.3.1), 1971). Lamps for general illumination purposes should emit light which can be considered as "white". White radiation is found in the color triangle at color points located on the Planckian locus or curve. This curve, which is also designated as the curve of the black body radiators and which is indicated hereinafter as the curve P, comprises the color points of the radiation emitted by a completely black body at different temperatures (the socalled color temperature). As the color temperature of white radiation is higher, the x co-ordinate and--from a temperature of about 2500 K.--also the y co-ordinate of the color point have a smaller value. A given color temperature is allotted not only to a given point on the curve P, but also to radiation having color co-ordinates located on a line intersecting the curve P at that point (see the said Publication CIE No. 15). If this radiation has a color point near the curve P, this radiation is also considered as white light having that given color temperature. In this description and the appended claims, the expression "a color point near the curve P" is to be understood to mean that the distance of the color point from the point on the curve P having the same color temperature is at most 20 MPCD (Minimum Perceptible Color Difference) is the unit of color difference; see the publication of J. J. Rennilson in "Optical Spectra", October 1980, page 63.
A large number of embodiments of low-pressure mercury vapour discharge lamps which have been known for tens of years and are still frequently used contain a luminescent alkaline earth metal halophosphate activated by Sb.sup.3+ and Mn.sup.2+. These lamps have the advantage that they are inexpensive and emit a satisfactorily high luminous flux. A great disadvantage of these lamps, however, is that their color rendition leaves much to be desired. They generally have R(a,8) values of the order of 50 to 60, and a R(a,8) value of about 75 is only attained with lamps having a high color temperature (for example, 5000 K.) which is not yet considered to be a satisfactory color rendition.
Lamps with which a very satisfactory color rendition is attained have been known for a long time. These lamps are provided with special luminescent materials, i.e. a tin-activated red-luminescing material on the basis of strontium orthophosphate, frequently in combination with a blue-emitting halophosphate activated by Sb.sup.3+, more particularly a strontium halophosphate of this kind. The said strontium orthophosphate luminesces in a very wide band which extends into the deep red. These known lamps have the disadvantage inherent to the use of the said strontium orthophosphate of a comparatively small luminous flux and of a poor maintenance of the luminous flux during the life of the lamp. It has been found that the last-mentioned disadvantage is a serious limitation to the use of this material in practice with a higher load by the radiation emitted by the mercury discharge.
A lamp of the kind described in the opening paragraph is known from German Patent Application No. 2,848,726. This lamp having a very satisfactory color rendition contains, as the aforementioned lamp type, a red-luminescing tin-activated strontium orthophosphate and further a borate phosphate which is activated by bivalent europium and has an emission band having a maximum at about 480 nm and a half-width value of about 85 nm. Preferably, a luminescent alkaline earth metal halophosphate is further used in the luminescent layer of this lamp. Due to the use of the luminescent strontium orthophosphate, this known lamp again has the disadvantages of a comparatively small luminous flux and more particularly of a poor maintenance of the luminous flux during the life of the lamp.
A satisfactory color rendition (R(a,8) values of 80-85), a high luminous flux and a good maintenance of the luminous flux, even with high loads, are obtained with lamps containing three luminescent materials emitting in three comparatively narrow bands (see Dutch Patent Specification No. 164,697). In these lamps, there is the drawback of disruption of the metamery of certain colors which, though of rare occurrence, is disturbing if particularly high requirements are imposed on the color rendition. The color impression of an object under the light of a lamp is then different from that under the light of a reference radiator having the same color temperature. The colors of two objects are generally designated as being metameric if these objects have a different reflection spectrum, but nevertheless give the same color impression under a given kind of light, for example, day-light. If under another kind of light, for example of an incandescent lamp, the color impression of these objects is different, the metamery is said to be disrupted.